A conventional type of plasticizer for injection molding machines has a screw with a helical thread that advances molten plastic along the length of a plasticizer barrel to an injection nozzle at the end of the plasticizer. These plasticizer screws have a shank portion and a helical thread extending beyond the shank portion with the outside diameter of the thread flights substantially equal to the entire diameter of the barrel. A substantial running clearance between the circumferential faces of the flights, and the inside surface of the barrel, is permissible because of the high viscosity of the plastic being conveyed.
The front surfaces of the screw flights propel the plasticized material forward with increasing pressure toward the discharge end of the screw.
Friction of the plastic material with the inside surface of the barrel and drag of the plastic on the front faces of the screw flights causes agitation and mixing of the plastic material as it advances along the barrel. There is, however, considerably less mixing, and sometimes none, in the regions near the juncture of the rearward surfaces of the flights with the shank portion of the screw. Plastic tends to remain in these regions, and with certain kind of plastic the part in these dead regions of the plasticizer sometimes decomposes and produces undesirable reaction products in the plastic that is being fed to an injection mold.
In order to assure mixing of the plastic material behind the flights, where dead regions in the plastic are likely to occur, this invention provides a groove in the shank portion immediately adjacent to the rearward face of the helical thread. This groove serves as a raceway for balls which extend above the shank portion of the screw to a height substantially equal to the circumferential surfaces of the flights of the screw thread.
This groove with the balls extends for a substantial portion of the length of the screw and there is an abutment at the forward end of the groove which causes the balls to enter a passage which extends inside the screw shank to an outlet end at the portion where the rearward end of the groove is located. As the screw rotates, these balls maintain agitation of the plastic in the region of the rearward faces of the screw thread flights.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.